It is the common practice in powered watercraft construction to provide a seal between the shaft and the hull to prevent water from flowing along the shaft into the hull. Inasmuch as the shaft rotates in either of two directions and moves axially in the forward or reverse direction when the thrust is reversed, the shaft seal must function efficiently under this quadridirectional movement of the shaft relative to the hull.
Shaft seals are also used in connection with rudder shafts, thrusters, stabilizers and the like to prevent leakage of water into the hull of the associated vessel.
In the past the most commonly used seal was an annular compression gland held under compression in a cavity of a body surrounding the shaft and mounted to the hull. The sealant material most commonly used has been hemp or cotton cordage impregnated with a lubricant. Such seals have been unreliable.
Shaft seals have been designed for other applications, but they are unsuitable for use with motorcraft and, therefore, have not found use therewith. For example, U.S. Pat. No. 4,095,808 discloses a shaft seal which permits for a very limited amount of axial movement of the shaft, much less than the one-half inch or more which is commonplace in powered watercraft. Also, that prior art shaft seal requires precise alignment of the shaft and the impeller relative to the housing, which would be the hull fitting if the seal were used in watercraft. Similar seals are disclosed in U.S. Pat. Nos. 4,363,491 and 4,732,396 where the seal assembly is maintained in fixed relationship with the housing.